I don't know if anyone is still checking, but I finally beat the last map (on easy difficulty - it was anything but easy!). Basically, I went with lots of fire towers and wind towers - often staggering them 1 after the other. I primarily placed them near the spawn points with the intention of only upgrading the fire towers all the way using the stake curse (the damage debuff, not the -armor one). This way, units are taking damage the entire time they're alive. Plus, many of the harder to kill units are most vunerable to fire if anything. The only thing I would have done differently was to put a lot more archer towers or rapid-fire cannons with the spike upgrade on the top-rightmost path, because in wave 24 (or 26) you get a lot of those fire-immune ghosts blazing down that path, and they take normal damage from cannon and an extra 50% from arrows. If you can survive wave 28 with at least 20-40 heath left, you're in good shape. On the last wave, as soon as the waves clear your first cannons' ranges near the spawn points, sell them and use the money to replace just about everything else with wind and fire towers where there weren't any before, due to the only enemies spawning being those shrewish valks. I didn't use any of the monoliths except for two on the bottom-left corner island, surrounded by rapid cannons. Odin likely won't be much of a worry, but try to have a fire tower or two on his path (he takes the top-right path down the right side) with the armor debuff, because his armor level is 9. Now ... to try it all again on normal difficulty...

Edit: Oh, hi everyone! First post and all that